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Welcome to AskUbuntu.com. Can you please give us more information about your graphics card and which drivers you are using? Add that to the question, it will be useful when trying to troubleshoot your issue.
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Bruno PereiraJan 3 '12 at 13:10

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On Ask Ubuntu, instead of writing [SOLVED] in the title of the question, it is better to leave the title so, to upvote the best answer and to mark it as the right one. That's a green check mark.
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AgmenorJan 23 '12 at 18:55

Thanks. I've resolved it myself. I went to CCSM(while in UNity 2d) and 'enabled' Unity plugin. After that I logged out and log into Unity 3d. This time it worked! :-)
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subeh.sharmaJan 3 '12 at 17:07

But another problem has arose, I use dual-monitors so I need to disconnect the 2nd monitor before starting Unity and then connect it back otherwise Unity will not load and i get stuck on some blank screen. :-(
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subeh.sharmaJan 5 '12 at 11:39